Efficiency of external combustion engines in a thermo-dynamic sense is a function of the difference between the rejection temperature to ambient and the maximum operating temperature. The magnesium/aluminum fuels and their hydrides, which are discussed in my prior patent and patent application identified above, can provide significantly higher maximum temperatures, and thus greater thermodynamic efficiencies. Materials and design of equipment generally determine the upper efficiency limit; for this reason I have invented a new and unique principle, which allows for higher operating temperatures, and satisfies my earlier requirement for collection and recycling of products of combustion, as well as the non-polluting aspects. The concept of a fluidized bed combustion process is introduced here to control and improve the combustion and to improve the overall heat transfer. A unique optional feature herein, is that the products of combustion (i.e. particles of various sizes, capable of withstanding high temperatures) themselves may be used at least in part to form a natural fluidized bed of solid particles to assist in the heat transfer. Combustible solids would be continuously introduced and excess products of combustion drawn off. In addition, the system could provide for preliminary partial separation of chemically different particles to aid in the recycling process.
Fluidized beds are old in the art and are designed on the principle of an upward vertical column of air flowing through a bed of solid particles, wherein the upward vertical velocity of air is sufficient to create an upward drag (lift) force on the particle which is approximately equal to the downward force of gravity on the particle. When this occurs the bed is said to be incipiently fluidized and the solid-gas mixture behaves to a large extent like a liquid. This results in high rates of heat transfer throughout the bed, high rates of solids mixing and of solids transport to and from the bed, reduction of temperature gradients due to a high degree of solids mixing, and high values of thermal inertia of the solids. Often the fluidized bed is precharged for a given function and recharged periodically, but the advantages of a continuous operation make this mode most desireable. Numerous technical papers are in the published literature on this subject so further expansion is not necessary.
The basis of the invention claimed herein is to utilize a fluidized bed as a combustion chamber for the combustion in an external combustion engine of the recyclable fuels magnesium, aluminum, their hydrides and/or alloys, and thereby capitalize on the thermodynamic advantages listed above. At the same time, excessively high flame temperatures can be reduced and controlled while still affording the higher efficiencies available at the generally elevated temperatures which exist within the combustion bed. These elevated temperatures would be limited in the practical case by materials, or by the working fluids. Fluidized beds generally utilize larger granules of an inert compound, such as silicon to promote mixing and control the fluid dynamics. Many higher temperature inert compounds, such as silicon carbide or titanium carbide, are available and contemplated for use with the elevated temperatures of the combustion process herein described.
A unique part of this invention, which advances the state of the art, is the use of the combustion products themselves in whole or in part as the inert media for the bed. It should be pointed out that the combustion products of the recyclable fuel basically will be magnesium oxide and aluminum oxide, and that these chemical compounds are often used as fire brick or furnace liners where high temperatures are involved; therefore, they become desireable as inert particle solids for the bed in the combustion process.
Further, it should be pointed out that while the flame temperatures of magnesium and aluminum are very high, the air to fuel ratios necessary for complete combustion are such that the particles will be immediately cooled from the gaseous (combustion) state and become solids while still in the fluidized bed; the reason being that when air is used to furnish the oxygen for combustion, an air to fuel ratio in excess of stoichiometric conditions is required for complete (efficient) combustion. This air to fuel ratio will typically yield gas flow temperatures considerably less than the flame temperatures. For example, with an air to fuel ratio of 20 to 1 (vice about 3.5 to 1 for stoichiometric conditions) for a magnesium-aluminum mixture, the flow temperature will be reduced to about 2400.degree. F. compared to the flame temperatures of aluminum (3700.degree. F.) and magnesium (5000.degree. F.). Higher temperatures may be obtained by either lowering the air to fuel ratio, or by introducing additional oxygen in a supplemental manner, such as introducing ammonia-perchlorate or potassium-perchlorate, into the combustion area. In this manner, gas flow temperatures greater than 2400.degree. F. may be obtained. Flow temperatures may be increased at least to 3700.degree. F., which is the melting point of Al.sub.2 O.sub.3. At this point and above, liquid aluminum oxide will exist within the bed. Although the introduction of a liquid state into the fluidized bed is not necessarily detrimental, it may complicate the mass transfer and thus become an "effective" maximum operating temperature. If however, liquids can be effectively accommodated in the solid-liquid-gas mixture, then the actual upper bound operating temperature would be about 5400.degree. F., the boiling (condensation) point for Al.sub.2 O.sub.3, the point at which capture of the gaseous products of combustion is no longer easily controlled.
The reason for this teaching is to establish an upper temperature limit on the fluidized bed, in keeping with the goal of maximum thermodynamic efficiency. As the container (crucible) for the fluidized bed may be constructed of ceramics, it must be capable of withstanding temperatures greater than 3700.degree. F.
I should also mention that, in my earlier patent and patent application on this subject, one of the preferred methods of transferring the heat of combustion to the working fluid of the engine was via sodium heat pipes. The phase change temperature of sodium, i.e., liquid to gas, would thus introduce another upper limit on the efficiency of the engine. Since there are other chemicals, such as lithium, with a higher phase change temperature, i.e., 2,418.degree. F., a chemical, such as this, may be used in the heat pipe and thus, lead to an increase in engine efficiency.
As a final point of background; when the fuel is combusted and heat extracted to perform useful work, the solid particles formed will be of various sizes, and hence assume a vertical distribution which is a function of their size. MgO and Al.sub.2 O.sub.3 particles of identical radius, however, will have different gravity forces, due to chemical composition, but identical drag forces.